Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. Among other things, lithium secondary batteries having a high-energy density and voltage, a long cycle lifespan and a low self-discharge rate are commercially available and widely used.
As cathode active materials for the lithium secondary batteries, lithium-containing cobalt oxide (LiCoO2) is largely used. In addition, consideration has been made to using lithium-containing manganese oxides such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure, and lithium-containing nickel oxides (LiNiO2).
Of the aforementioned cathode active materials, LiCoO2 is currently widely used due to superior general properties including excellent cycle characteristics, but suffers from low safety, expensiveness due to finite resources of cobalt as a raw material, and limitations in practical and mass application thereof as a power source for electric vehicles (EVs) and the like.
Lithium manganese oxides, such as LiMnO2 and LiMn2O4, are abundant resources as raw materials and advantageously employ environmentally-friendly manganese, and therefore have attracted a great deal of attention as a cathode active material capable of substituting LiCoO2. However, these lithium manganese oxides suffer from shortcomings such as low capacity and poor cycle characteristics.
Whereas, lithium/nickel-based oxides including LiNiO2 are inexpensive as compared to the aforementioned cobalt-based oxides and exhibit a high discharge capacity upon charging to 4.3 V. The reversible capacity of doped LiNiO2 approximates about 200 mAh/g which exceeds the capacity of LiCoO2 (about 165 mAh/g). Therefore, despite a slightly lower average discharge voltage and a slightly lower volumetric density, commercial batteries comprising LiNiO2 as the cathode active material exhibit an improved energy density. To this end, a great deal of intensive research is being actively undertaken on the feasibility of applications of such nickel-based cathode active materials for the development of high-capacity batteries. However, the LiNiO2-based cathode active materials suffer from some limitations in practical application thereof, due to the following problems.
First, LiNiO2-based oxides undergo sharp phase transition of the crystal structure with volumetric changes accompanied by repeated charge/discharge cycling, and thereby may suffer from cracking of particles or formation of voids in grain boundaries. Consequently, intercalation/deintercalation of lithium ions may be hindered to increase the polarization resistance, thereby resulting in deterioration of the charge/discharge performance. In order to prevent such problems, conventional prior arts attempted to prepare a LiNiO2-based oxide by adding an excess of a Li source and reacting reaction components under an oxygen atmosphere. However, the thus-prepared cathode active material, under the charged state, undergoes structural swelling and destabilization due to the repulsive force between oxygen atoms, and suffers from problems of severe deterioration in cycle characteristics due to repeated charge/discharge cycles.
Second, LiNiO2 has shortcomings associated with evolution of excess of gas during storage or cycling. That is, in order to smoothly form the crystal structure, an excess of a Li source is added during manufacturing of the LiNiO2-based oxide, followed by heat treatment. As a result, water-soluble bases including Li2CO3 and LiOH reaction residues remain between primary particles and thereby they decompose or react with electrolytes to thereby produce CO2 gas, upon charging. Further, LiNiO2 particles have an agglomerate secondary particle structure in which primary particles are agglomerated to form secondary particles and consequently a contact area with the electrolyte further increases to result in severe evolution of CO2 gas, which in turn unfortunately leads to the occurrence of battery swelling and deterioration of desirable high-temperature safety.
Third, LiNiO2 suffers from a sharp decrease in the chemical resistance of a surface thereof upon exposure to air and moisture, and gelation of slurries by polymerization of an N-methylpyrrolidone/poly(vinylidene fluoride) (NMP-PVDF) slurry due to a high pH value. These properties of LiNiO2 cause severe processing problems during battery production.
Fourth, high-quality LiNiO2 cannot be produced by a simple solid-state reaction as is used in the production of LiCoO2, and LiNiMeO2 (Me=Co, Mn or Al) cathode active materials containing an essential dopant cobalt and further dopants manganese and aluminum are produced by reacting a lithium source such as LiOH·H2O with a mixed transition metal hydroxide under an oxygen or syngas atmosphere (i.e., a CO2-deficient atmosphere), which consequently increases production costs. Further, when an additional step, such as intermediary washing or coating, is included to remove impurities in the production of LiNiO2, this leads to a further increase in production costs.
Further, Japanese Unexamined Patent Publication Nos. 2004-281253, 2005-150057 and 2005-310744 disclose oxides having a composition formula of LiaMnxNiyMzO2 (M=Co or Al, 1≦a≦1.2, 0≦x≦0.65, 0.35≦y≦1, 0≦z≦0.65, and x+y+z=1). However, it was found through various experiments conducted by the inventors of the present invention that the aforementioned oxides include large amounts of impurities such as lithium carbonates, and suffer from significant problems associated with severe gas evolution at high temperatures and structural instability.
Therefore, there is an urgent need in the art for the development of a technology which is capable of achieving a structurally stable crystal structure while utilizing a lithium/nickel-based cathode active material which can allow for high charge capacity and is capable of securing high-temperature safety.